In recent years, communication systems that notify vehicles that are running of safe driving support information that represents, for example, neighboring road conditions and neighboring vehicle running states through road-to-vehicle communication (between a road side unit and the vehicle itself) and inter vehicle communication (between a vehicle itself and another vehicle) have been implemented so as to reduce traffic accidents.
A vehicle of a transmission source that is an information transmission source (hereinafter referred to as “transmission vehicle”) often provides the safe driving support information used for these communication systems in one way to neighboring vehicles that are in a predetermined direction of and apart from the transmission source vehicle. For example, an emergency vehicle information provision support system expected to be one application of the inter vehicle communication provides information to vehicles that are present in an area 300 meters in front of an emergency vehicle that is a transmission vehicle.
In this communication system, microwaves are mainly used. However, high frequency radio waves such as microwaves have features in which the propagation distances are short and the diffraction losses are large. Thus, it may occasionally be difficult for a transmission vehicle to directly communicate with vehicles that are present in a predetermined transmission information target area (hereinafter referred to as “transmission destination area”).
FIG. 1 is a schematic diagram showing a technique to enlarge the transmission destination area for a transmission vehicle. As shown in FIG. 1, vehicle B forwards transmission information transmitted from transmission vehicle A so as to enlarge a transmission destination area (hereinafter vehicle B is referred to as “forwarding vehicle”).
When forwarding vehicle B receives a packet containing transmission information from transmission vehicle A, forwarding vehicle B also transmits the packet to the outside of the transmission destination area. If forwarding vehicle B repeats wasteful forwarding of the packet, communication traffic may increase and information may not be adequately transmitted.
On the other hand, the geocast routing technique that restricts an area to which a transmission packet is forwarded is known. FIG. 2 and FIG. 3 show examples of areas specified by the geocast routing technique.
FIG. 2 is a conceptual diagram in which a specified square area is set up using four-point position coordinates according to the geocast routing technique. FIG. 3 is a conceptual diagram in which a specified circular area is set up using center coordinates and a radius according to the geocast routing technique.
Patent Literature 1 describes an example of a radio network system according to the geocast routing technique that sets up a specified area using parameters that represent the forwarding direction of a packet and the effective width of an area to which the packet is forwarded.
In the radio network system described in Patent Literature 1, a terminal unit that is a transmission source uses the parameters, which represent the forwarding direction of the packet and the effective width of the area to which the packet is forwarded (hereinafter these parameters are referred to as “specified parameters”), adds the parameters to the packet, and transmits the packet. When a forwarding terminal unit receives the packet from the terminal unit that is the transmission source, the forwarding terminal unit determines whether or not its current position is present in the specified area that corresponds to the specified parameters. If the current position of the forwarding terminal unit is not present in the specified area, the forwarding terminal unit does not forward the packet.
Thus, this radio network system can decrease the number of forwarding times that forwarding vehicles forward the packet and thereby reduce of packet transmission congestion.